Communication systems use a variety of methods to encode signals and increase bandwidth. Orthogonal Frequency Division Multiplexing (OFDM) is a form of signal modulation using a large number of parallel narrow-band subcarriers. However, OFDM is sensitive to sampling frequency offset.
FIG. 1 shows an OFDM waveform with no sampling frequency offset. Subcarriers 10 carry modulated symbols for the data being transmitted. Other signals 12 include background noise and harmonics but tend to cancel out, allowing subcarriers 10 to be detectable by a receiver.
Ideally, a receiver samples each of subcarriers 10 near the subcarrier's peak. For example, first subcarrier C1 is sampled by a receiver clock near its maximum, second subcarrier C2 is sampled near its maximum, third subcarrier C3 is sampled near its peak, and last subcarrier CN is sampled near its peak. An oscillator in the receiver causes the receiver data extraction circuitry to sample the received waveform periodically. The frequency of this receiver clock oscillator needs to be carefully adjusted so that sampling occurs near the peaks of subcarriers 10.
FIG. 2 shows an OFDM waveform with a sampling frequency offset. In this example, a small error in the receiver sampling-clock oscillator is present. The period of the receiver clock is elongated, causing the receiver sampling clock to be too slow. The first sampling edge of the receiver clock occurs near the peak of the first subcarrier C1, so a high signal S1 is detected for C1. However, a slight error in the sampling clock causes the next sampling edge to occur a little after the peak of second subcarrier C2. The signal S2 for carrier C2 is slightly lower than for C1.
The sampling errors are cumulative, so the error for the third sampling edge is greater. Third subcarrier C3 is sampled as the signal is falling, and its sampled signal strength S3 is less than S1 or S2. Similarly, sample points for subcarriers C4, C5, C6 are delayed further, causing sampling to occur farther and farther down from their peaks, at signal strengths S4, S5, S6.
At last sampling point 16 for subcarrier C7, that subcarrier's signal has fallen so far below its peak that the signal strength S7 is within the noise of other signals 12 and is no longer able to be read correctly. While the receiver clock was accurately synchronized to first subcarrier C1 at first sampling point 14, the slight errors accumulate for each successive subcarrier 10, causing increasing errors in sampling, until the last subcarrier is unreadable.
Sampling Frequency Offset (SFO) is caused by clock mismatch between the transmitter and the receiver. Clock mismatch always exists between two oscillators. When SFO exists, orthogonality is reduced and data errors increase. Sampling discrepancy grows with each carrier away from the first carrier. Even small levels of discrepancy cause the error rate to increase.
To overcome these drawbacks of SFO, Sampling Frequency Offset Estimation (SFOE) may be used for synchronization of an OFDM receiver. Most SFOE methods rely on a preamble and pilots. The preamble is located at the beginning of the frame that is used for synchronization. The preamble can be used in SFO estimation. Pilots are distributed within the payload. Pilots are commonly used for tracking residual frequency offsets.
FIG. 3 shows pilots in an OFDM frame. OFDM frame 20 includes data blocks 22 when data is transmitted, and pilots 24. Pilots 24 are staggered so that any subcarrier frequency (x-axis) has a pilot at that frequency once every 4 frequency slots. Pilots 24 contain a pattern that allows the receiver's sampling clock to be re-synchronized, allowing clock synchronization errors to be reset. While such pilots are useful, some communications standards do not use pilots.
Another approach is to use the cyclic prefix that precedes the OFDM symbols. These cyclic prefixes can be used for SFO estimation. However, the performance is limited by the cyclic prefix length and by channel reflections.
Some communication standard place extremely high requirements on SFO. For example, the HomePlug AV2 standard specifies high order modulation and long frame length, and a high sampling frequency. However, a preamble based SFOE cannot offer enough accuracy for the estimation. Moreover, there are no pilot symbols distributed in the payload for tracking the SFO when using HomePlug power line applications.
FIG. 4 shows a typical OFDM structure for the HomePlug AV2 standard. OFDM structure 30 starts with AV2 preamble 26, which is 10K in length. Preamble 26 has a fixed pattern of high and low signals that does not occur in data payloads.
The first symbol, AV2 Frame Control (FC) symbol 35, is preceded by Guard Interval (GI) 31. Each symbol 35, 36, 37, 38, is preceded by a Guard Interval 31, 33, 34 that ensures that symbols do not interfere with other symbols. Overlapping transmission is prevented by the guard intervals. Echoes from the previous symbol may fall within the guard interval and still not interfere with the subsequent symbol.
Frame control symbol 35 provides frame control information, such as the number and length of subsequent symbols 36, 37, 38. Frame control symbol 35 may contain information about the physical block size, number of symbols, tone map information, mapping type, FEC rate, etc.
Symbols 36, 37, 38 are data symbols that carry the data payload. These symbols can be 8192 multi-bit samples long, while Guard Intervals 32, 33, 34 are 1512 samples long. Many more symbols than shown are typically present, each with its own guard interval.
Although the HomePlug AV2 standard provides OFDM structure 30 with Guard Intervals 31, 33, 34 and frame control symbol 35, there are no pilots. Symbols 36, 37, 38 carry data but no pilots. Thus Sampling Frequency Offset (SFO) techniques that use pilots cannot be used with the HomePlug AV2 standard.
What is desired is a method establishing a feedback loop for tracking sampling frequency offset for an OFDM receiver. A SFO compensation circuit that increases the efficiency and accuracy of the SFO is desirable. A SFOE circuit for communication standards that do not have pilots is desired. A SFOE circuit for the HomePlug AV2 standard is desired.